1. Field of Invention
The present invention relates to an antimicrobial composite material and a method for producing the same. More particularly, the present invention relates to a single or multiple metal vermiculite composite material having antibacterial/antifungal activities, and to the method of making and using the same.
2. Discussion of Prior Art
Vermiculite is a naturally occurring layer-structured mineral, which is in the size of 0.1 mm to several centimeters, and composed of alternative aluminum silicate sheets and hydrate cation sheets. The cations in the interlayer region are exchangeable and can be easily replaced in the laboratory by cation exchange reaction. Within the interlayer regions of vermiculite, water molecules will transform into steam when rapidly heated to high temperature (around 850° C. or higher), causing the dramatic increase of volume of vermiculite particles (expansion, usually called exfoliation). Exfoliated vermiculite has very unique properties, such as high porosity, high specific surface area, high aspect ratio of laminates, low density, and high cation exchange capacity. Typically, the cation exchange capacity of vermiculite reach 50-150 mmol/100 g.
Vermiculite is extensively distributed in the United States and worldwide. Annually, over 100 thousand tons of vermiculite was produced, and over 150 thousand tons of vermiculite was consumed in the United States. Vermiculite has been extensively used in various industrial products and applications. Most vermiculite is consumed in thermally exfoliated form. These applications include agricultural growth media, lightweight aggregates, building boards, insulations, additives in coatings, plastics, fertilizers, papers, and wood products, etc. However, there is no report on the making of antimicrobial exfoliated vermiculite.
Currently, there have been some reports on making antimicrobial additive with clay minerals, such as montmorillonite, bentonite, kaolinite. Clay minerals have similar silicate sheets and cation exchange property with vermiculite. However, vermiculite is significantly distinguished itself from clay minerals by its natures, such as unique interlayer cations, high layer charge that results in a high cation exchange capacity, unique exfoliation function, and a larger particle size. Further, exfoliated vermiculite has unique properties, such as lightweight, high porosity, and high surface absorption.
It has been found that some transition metals, such as silver, copper, zinc, nickel, and manganese, exhibit antimicrobial activities. Numerous attempts have been made to utilize this property to various applications. For example, U.S. Pat. No. 5,009,898 describes antimicrobial hydroxyapatite powders containing hydroxyapatite powder and metal ions selected from silver, copper and zinc ions.
U.S. Pat. No. 5,441,717 describes process for producing antimicrobial compounds by forming metal salts of silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium.
U.S. Pat. No. 5,503,840 exposures an antimicrobial composition containing titanium dioxide, barium sulfate, zinc oxide particles, and mixtures thereof having successive coatings of silver, in some cases a coating of zinc and/or copper compounds such as zinc oxide, copper (II) oxide and zinc silicate; silicon dioxide; alumina. This composition made up of polymer, which also is the host of metals.
U.S. Pat. No. 6,143,318 describes an antimicrobial composition containing copper, silver, magnesium, zinc, cerium, manganese bismuth, selenium and boron. In this composition, antimicrobial metals were held by glass.
U.S. Pat. No. 5,298,253, disclosed a granular copper hydroxide dry flowable bactericide/fungicide. In this process, copper hydroxide was admixed with bontonite, and dried granules were formed by spray drying method.
U.S. Pat. No. 4,497,869 describes formed product of vermiculite (bulk density of from 0.9 to 2.5 (g/cm.sup.3)) containing metal salts by mixing process, to improve the water resistance and strength of products.
U.S. Pat. No. 4,210,530 describes the treatment of metal plating wastes with an unexpanded vermiculite cation exchange column, which employs unexfoliated vermiculite as an absorption to filter heavy metals such as copper.
Summarily, there is no report on preparation and application of exfoliated vermiculite antimicrobial compound. In addition, only ionic metals and their nanoparticles perform antimicrobial activity. These metallic ions also need an effective carrier for their loading, desirable release rate, chemical stability, and durability for their appropriate uses. How to hold and release the ionic metals appropriately is a significant issue to the development and applications of metal ion-typed antimicrobial materials. Vermiculite can provide a consistent and effective delivery vehicle for antimicrobial agents. Metal ions in the interlayer of vermiculite can be slowly released via cation exchange and delivered to the surface of particles by diffusion.